I have never heard of a condensation paint.Paint will only seal the paper from penitrating damp not condensation. The moisture occurs from the heat in the room and persons exhalling warm air,conflicting with the cold exterior, the same as in a tent.The air vent you have is obviously not working,so either its blocked or you need to install another.
The thing is, air vents don't always work as you might expect them to. An air vent will only work when there is another hole somewhere in the home. Then depending on the wind direction and speed the air may be moving out or in or not moving at all. Water vapour is programmed by nature to move towards the nearest cold surface or space. If you have condensation on a wall, it is because the wall is cold. And that somewhere you have a source of warm wet air. This may just be someone in the room. It may be from a nearby bathroom...the doors being open? Or the kitchen? If you warm the wall the condensation will move somewhere else, probably a window. It may be that the air vent is making the wall cold and is causing the problem. It is never a good idea to have holes in your home as the passing wind will suck your warm our out. It is better to deal with stale air and water vapour by opening a window, doing this you can judge how strong the wind is, how cold it is outside, how much it is costing you and close it when satisfied.
perry 525 is correct about the condensation issue, and if the vent is not working and your external wall is that cold,its best to insulate it.But I think that trickle vents in the window will have to be installed.